1. Field of the Invention
The present invention relates to an improved ceramic composition for voltage non-linear resistor which comprises zinc oxide as a main component and components of praseodymium, lanthanum and cobalt and an additional component as minor components. More particularly, it relates to a sintered body of a ceramic composition for voltage non-linear resistor which has remarkably large voltage non-linearity and large discharge capacity.
2. Description of the Prior Art
Recently, ceramic non-linear resistors (hereinafter referring to as ceramic varistors) having excellent voltage non-linearity characteristics comprising zinc oxide as a main component have been widely used as electronic parts for protection of a circuit and prevention of erroneous operation. Varistors having excellent voltage non-linearity in a large current region have been also required. In voltage-ampere characteristic of a varistor, a current to a voltage is non-linearity varied as shown in the FIGURE. Thus, the voltage-ampere characteristic of a varistor is usually shown by the equation: EQU I=(V/C).sup..alpha.
wherein I designates current passed through the varistor; V designates a voltage applied to the varistor; C designates a constant corresponding to the resistance; and .alpha. designates an index for a non-linearity. A voltage for passing a current of 1 mA is usually referred to as a varistor voltage.
In a broad current region, .alpha. is varied depending upon the voltage. When the non-linearity in a wide current region, it is preferable to consider a ratio of a voltage in the low current region to a voltage in the large current region, for example, a ratio of V.sub.1mA to V.sub.50A shown in the FIGURE. The voltage non-linearity characteristic is superior depending upon lower voltage ratio.
Recently, ceramic varistors as a combination of an electrode and a ceramic comprising zinc oxide as a main component and oxides of bithmuth, antimony, manganese, cobalt and chromium as minor components have been developed. The voltage non-linearity of such ceramic varistor is resulted by the characteristics of the sintered composition. The non-linearity is advantageously remarkable in the wide current range. Thus, on the other hand, the composition comprises components which are easily volatilized at high temperature required for sintering a composition for the varistor, such as bismuth and antimony. It is necessary to consider special conditions for sintering compositions in a mass production so as to produce varistors having the same characteristic at a low ratio of defective products whereby the production cost hs been remarkably high.
On the other hand, ceramic varistors as a combination of an electrode and a ceramic comprising zinc oxide as a main component and components of oxides of praseodymium, cobalt, chromium and potassium have been also developed (Japanese Unexamined Patent Publication No. 114093/1978). These ceramic varistors do not contain volatile components such as bithmuth and antimony components, and have excellent voltage non-linearity, however, it is necessary to incorporate, potassium and chromium components so as to improve voltage non-linearity characteristics in the large current region. Thus, the incorporation of potassium causes the serious problem of low moisture resistance as electronic parts. In the practical application of such ceramic varistor, it is necessary to protect such ceramic varistor by coating the surface of the sintered ceramic varistor with a molten glass whereby steps for the production are disadvantageously increased and a cost for production is disadvantageously high. Moreover, a relatively large quantity of praseodymium having high purity is needed, though the source of praseodymium is not large enough. This is disadvantageously uneconomical.